It wasn’t until late in the Enterprise-D’s design process that Andrew Probert learned the ship would need to be able to split in two.
The ability reflected Gene Roddenberry’s thinking about The Next Generation. Unlike in the original Star Trek, this Enterprise would have families aboard.
“Just because you’re on a space exploration, it doesn’t mean you have to give up your life,” Executive Producer Robert Justman is quoted as saying in the Star Trek: The Next Generation USS Enterprise NCC-1701-D Blueprints.
If you have loved ones, you have family, you should be able to enjoy and live and make your lives together, even though you’re out in space. […] For that we provided for the separation capabilities, so that in case of danger or threat to the ship as a whole, if necessary we would send the saucer section away under its own power.
That’s not what Probert heard initially. He thought the producers wanted a battle section to separate from the rest of the ship as opposed to leaving the saucer behind.
“I thought maybe it was like an auxiliary craft or something that separated to go off and fight battles,” he told Star Trek: The Magazine in 2000.
That’s why his first designs had a D-shaped section separating from the saucer. The pylons would be the warp engines.
That’s not what the producers had in mind:
When I showed this to them they said, “No, no. What’ll happen is the saucer separates and the engineering hull then becomes the battle section.” So I have this dorsal sculpted into the saucer and now I have to separate that and still make it look good both ways, which was an extraordinary challenge.
Probert decided part of the underside of the saucer should remain with the engineering hull, giving it a cobra-like head with the battle bridge on top.
Another option was to separate several parts of the ship. The smaller sections would have been large lifeboats. Something like this was done on Star Trek: Voyager with the USS Prometheus.
Saucer separation was meant to have been a regular feature on The Next Generation, but budget restraints did not allow it to happen often. Writers also felt separating the ship slowed the progress of the story. As a result, the original idea — protecting families — was seldom realized.